Have a loook with the extended Tag-View (ALT-t).
You will see that there are 2 year-tag-fields.
In the extended tag-view you can delete one of these.
There is also an action of the type "Remove Duplicate Fields".
A possible reason why this happened maybe the use of Mediamonkey for tagging.
If you use Mediamonkey and want to use it also in the future: there is a setting in Mediamonkey that can avoid this.
Add this to the file mediamonkey.ini:
[MP3Tagging]
DisableFrames=TDRC;TDOR;
EnableFrames=TYER;TDAT;TORY;